Natarajan et al. in Belgian Patent No. 897,327 disclose amino ketone dipeptides of the formula ##STR2## wherein R.sub.1 is hydrogen only when R is other than hydrogen.
Gordon et al. in U.S. Ser. No. 515,729 filed July 21, 1983 disclose hydroxy substituted dipeptides of the formula ##STR3## which are prepared by treating the corresponding keto compound with a reducing agent.
Meyer et al., "Angiotensin Converting Enzyme Inhibitors: Modifications of A Tripeptide Analogue", J. Med. Chem., 1982, 25, 996-999, disclose the synthesis and angiotensin converting enzyme inhibition activity of compounds of the formula ##STR4## wherein X can be NH and R can be L-proline.
Almquist et al. in U.S. Pat. No. 4,329,473 disclose oxoalkanoic acid derivatives of L-proline as angiotensin converting enzyme inhibitors.